Fig. 4. (A,B) Spontaneous action potentials in the two olfactory receptor neurons (ORNs) in a trichoid sensillum, distinguishable by their amplitudes. The three parts in A are one continuous sequence. The action potentials of neither class were randomly distributed, but (both) exhibited bursting activity. Bursts of large action potentials are marked with filled circles, bursts of small action potentials with open circles. The indicated burst (star) is shown on an enlarged time scale in B. (C) A burst of large action potentials from a different recording, illustrating the reduction in amplitude of the action potentials. (D) Action potential bursts were also recorded with tungsten electrodes placed near the hair base. The three parts of D are one continuous trace, highpass-filtered at 5 Hz.